1. Field of the Invention
The present invention relates to an over-current protection device and, more particularly, to an over-current protection device comprising a positive temperature coefficient (PTC) conductive material. The over-current protection device presents better resistivity and resistance repeatability, especially suitable to the protection of a power source used in portable communication applications.
2. Description of the Prior Art
The resistance of PTC conductive material is sensitive to temperature change. With this property, the PTC conductive material can be used as current-sensing material and has been widely used in over-current protection devices and circuits. The resistance of the PTC conductive material remains at a low value at room temperature so that the over-current protection device or circuits can operate normally. However, if an over-current or an over-temperature situation occurs, the resistance of the PTC conductive material will immediately increase at least ten thousand times (over 104 ohm) to a high-resistance state. Therefore, the over-current will be counterchecked and the objective of protecting the circuit elements or batteries is achieved.
In general, the PTC conductive material contains at least one crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer(s). The crystalline polymer is mainly a polyolefin polymer such as polyethylene. The conductive filler(s) is mainly carbon black, metal particles and/or non-oxide ceramic powder, for example, titanium carbide or tungsten carbide.
The conductivity of the PTC conductive material depends on the content and type of the conductive fillers. Generally speaking, carbon black having a rough surface provides better adhesion with the polyolefin polymer, and accordingly, a better resistance repeatability is achieved. However, the conductivity of the carbon black is lower than that of the metal particles. If the metal particles are used as the conductive filler, their larger particle size results in less uniform dispersion, and they are prone to be oxidized, which causes high resistance. To effectively reduce the resistance of the over-current protection device and prevent oxidation, the ceramic powder tends to be used as the conductive filler in a low-resistance PTC conductive material. Since it lacks a rough surface like carbon black, the ceramic powder exhibits poor adhesion with the polyolefin polymer, and consequently, the resistance repeatability of the PTC conductive material is not well controlled. In prior arts, to improve the adhesion between the metal particles and the polyolefin polymer, a coupling agent will be added into the conventional PTC conductive material with the ceramic powder as the conductive filler. The coupling agent may be an anhydride compound or a silane compound. However, the total resistance of the PTC conductive material after the coupling agent is added cannot be reduced effectively.
Currently, a low-resistance (about 20 mΩ) PTC conductive material with nickel as the conductive filler is available in the public market, but it can only sustain a voltage up to 6V. If the nickel is not isolated well from the air, it is prone to be oxidized after a period, and this results in increasing resistance. In addition, the resistance repeatability of the low-resistance PTC conductive material is not satisfied after tripping.